The present invention relates to the protection of vessels and equipment, particularly static equipment such as drill ships, moored in polar regions. Such installations are vulnerable to damage from moving ice, especially pack ice, which comprises a fairly dense array of ice structures, up to about 100 million tonnes in mass, linked by sheet ice.
Various proposals have been made with a view to protecting installations from pack ice. The proposals have in common the idea of enabling installations to withstand the impact of ice moving at its normal speed. For example, one proposal is to support a production platform on a pillar which gradually widens beneath the surface of the water into a massive supporting base, restng on the water-bed. However, such a structure, known as a monocone, cannot resist a large ice structure and once off site cannot generally be returned until the pack ice thaws the following summer.
Another proposal is to build an artificial sand or gravel island on which to site the drilling equipment. Such an island would need to be massive to withstand the largest ice structures, would be difficult and expensive to construct, and would be subject to erosion.
A third proposal is to use a steel or concrete caisson filled with sand or gravel, sitting on a small artificial gravel island. However, such equipment could not feasibly be used in deep water.
According to a first aspect of the present invention there is provided apparatus for protecting an installation from floating ice moving towards the installation, comprising:
an array of buoyant drag elements upstream of the installation; PA1 a number of underwater anchorages upstream of the installation; PA1 anchor lines connecting the drag elements to the anchorages; PA1 and means at the surface of the drag elements for engaging the under surface of ice moving over the drag elements, whereby the ice is decelerated.
The terms upstream and downstream in this specification refer to the directions of movement of the ice, as determined by the current and the wind.
The present invention seeks to overcome the problems arising from moving ice by steadily decelerating the ice as it passes over the drag elements, rather than by buttressing the installation.
In a preferred embodiment the means at the surface of the drag elements for engaging the under surface of the ice comprise teeth which, urged upwards by the drag elements, penetrate the ice. Such teeth are preferably three-sided in transverse cross section and have a point at their leading edge, to penetrate the ice. The teeth may be arranged in transversely staggered, longitudinally spaced, rows, such that no tooth follows the line of another tooth.
Preferably the drag elements form a generally circular array around the installation which is to be protected, so that the installation may be protected from ice moving in any direction. The number, size and distribution of elements is such that when ice has finished moving over drag elements its momentum is too low for it to cause any damage to the installation. The drag elements may thus enclose a protection area around the isntallation. As the ice is slowed down by the drag elements a wedge of relatively densely packed ice forms upstream of the protection area. The wedge tends to deflect further oncoming ice around the protection area.
According to a second aspect of the invention there is provided a structure having teeth projecting from its surface and being floatable in water, and connectable to an undercover anchorage.
Such a structure can act as a drag element as previously described.